1. Field of the Invention
Aspects of the present invention relate to a membrane electrode assembly for a fuel cell and a method of manufacturing the same, and more particularly, to a membrane electrode assembly including a porous catalyst layer adjacent to a surface of an electrolyte membrane of the membrane electrode assembly for a fuel cell and a method of manufacturing the same.
2. Description of the Related Art
Fuel cells are devices in which chemical energy is converted into electrical energy through electrochemical reaction of a fuel with oxygen. Fuel cells theoretically have very high power generation efficiency since they are not based on the Carnot cycle. Such fuel cells can be used as power sources for compact electric/electronic devices, particularly portable devices, as well as for industrial, domestic, and transportation applications.
Fuel cells are classified into polymer electrolyte membrane (PEM) fuel cells, phosphoric acid fuel cells, molten carbonate fuel cells, solid oxide fuel cells, etc., according to the type of electrolyte used. The operating temperature of the fuel cell and the materials therein vary depending on the electrolyte used.
The fuel cell can be further classified according to how the fuel is fed, including an exterior reforming type that converts a fuel to a hydrogen enriched gas through a fuel reformer and feeds the hydrogen enriched gas to an anode of the fuel cell, a direct fuel feeding type that directly feeds a fuel in a gas or a liquid state to an anode, or an interior reforming type.
A representative example of the direct fuel feeding type is a direct methanol fuel cell (DMFC). In the DMFC, an aqueous methanol solution or a mixed vapor of methanol and water is generally fed to an anode. DMFCs do not require an external reformer and use fuel that is convenient to handle, and DMFCs have the highest potential for use as portable energy sources.
Electrochemical reactions occurring in a DMFC include an anode reaction in which fuel is oxidized and a cathode reaction in which oxygen is reduced into water through a reaction with protons from the oxidized fuel, and the reactions are as follows.                Anode Reaction: CH3OH+H2O→6 H++6 e−+CO2         Cathode Reaction: 1.5 O2+6 H++6 e−→3H2O        Overall Reaction: CH3OH+1.5 O2→2 H2O+CO2         
As shown in the reaction schemes, one methanol molecule reacts with one water molecule at the anode to produce one carbon dioxide molecule, six protons, and six electrons. The produced protons migrate to the cathode through a proton conductive electrolyte membrane. The protons react with oxygen and electrons which are supplied via an external circuit in the cathode to produce water. In the overall reaction in the DMFC, water and carbon dioxide are produced through the reaction of methanol with oxygen. As a result, a substantial portion of the energy equivalent to the heat of combustion of methanol is converted into electrical energy. The anode and the cathode include catalysts to facilitate those reactions.
The proton conductive electrolyte membrane provides a path for the protons generated through the oxidation reaction at the anode to migrate to the cathode, and electrically separates the anode and the cathode. Generally, the proton conductive electrolyte membrane is hydrophilic, and thus the proton conductive electrolyte membrane is generally impregnated with an appropriate amount of water to increase the ionic conductivity thereof.
A portion of methanol that is fed to the anode is diffused to the hydrophilic proton conductive electrolyte membrane and migrates to the cathode. Such migration of methanol is a methanol cross-over. Typically, the cathode of the DMFC includes a platinum catalyst which facilitates oxidation of methanol as well as reduction of oxygen. Thus, the crossed-over methanol is oxidized, and accordingly, performance of the DMFC considerably decreases.
In order to overcome methanol cross-over, efforts to develop a proton conductive electrolyte membrane capable of preventing methanol permeation and a cathode catalyst having low reactivity with methanol have been made. Further, the cathode catalyst layer needs to have capability of transferring oxygen and effectively removing water.
To improve oxygen adsorbing capability, pores in the cathode catalyst layer should be small and overall porosity of the cathode catalyst layer should be increased. However, when the pore size is too small, water is not easily removed in the catalyst layer. On the other hand, when the pore size is too large, overall porosity decreases, and thus oxygen adsorbing capability decreases even though water is easily removed. Therefore, pore size and porosity are required to transfer oxygen and effectively remove water.
To prepare a cathode catalyst layer having such properties, Japanese Patent Publication No. 2006-147371 discloses a method of preparing a catalyst layer having two types of pore sizes by simultaneously sputtering Pt and Fe particles and then removing Fe using hydrochloric acid.
Conventionally, an electrolyte catalyst for a high output fuel cell has been developed by feeding reactant gases and effectively discharging produced water by preparing a catalyst layer having various pore sizes.